Heretofore, carbon black is frequently used as a filler for rubber reinforcement. Because, the carbon black can be given a high reinforcing property and an excellent wear resistance as compared with the other fillers. On the other hand, with the escalation in the recent social demand on energy saving and in the interest on the environmental problem are more increasing the attention of low fuel consumption in automobiles and the demand on the low heat buildup in the rubber composition for tires accompanied therewith. As a countermeasure is considered the decrease of the filling amount of carbon black or the use of large particle size carbon black. In these cases, however, there is known that it is unavoidable to lower the reinforcing property, wear resistance and gripping property on wet road surface. On the contrary, silica hydride (wet silica) is known as a filler for establishing the low heat buildup and the gripping property on wet road surface (see, for example, JP-A-3-252431, JP-A-6-248116, JP-A-7-70369, JP-A-8-245838 and JP-A-8-337687), and many studies and developments are made.
However, silica has a tendency that particles are aggregated to each other due to hydrogen bond of silanol group as a surface functional group, so that it is required to prolong the milling time for improving the dispersion of silica particles into rubber. Also, the dispersion of silica particles into rubber is insufficient, so that there are drawbacks that the Moony viscosity of the rubber composition becomes high and the processability in the extrusion or the like is poor. Furthermore, since the surfaces of silica particles are acidic, they have a drawback that a basic substance used as a vulcanization accelerator is adsorbed to conduct the insufficient vulcanization of the rubber composition and hence the elastic modulus is not increased. In order to solve the above-drawbacks, silane coupling agents are developed. In the latter case, however, the dispersion of silica particles does not arrive at a sufficient level, and particularly it is difficult to industrially obtain the good dispersion of silica particles.
In order to provide rubber compositions having a low heat buildup, there have hitherto been made many technological developments for enhancing the dispersibility of the filler used in the rubber composition. Among them is most popularly made a method wherein a polymeric activating end of a diene polymer obtained an anion polymerization using an organolithium compound, particularly styrene-butadiene copolymer is modified with a functional group having an interaction with the filler. Under the aforementioned situations, it is variously attempted to apply this technique to a system using carbon black as a reinforcing filler.
For example, there are mentioned a method wherein carbon black is used as a reinforcing filler and the polymeric activating end is modified with a tin compound such as tin tetrachloride or the like (see JP-B-5-87530), a method wherein carbon black is used likewise the above and both the polymeric activating ends are modified with the tin compound (see JP-A-6-49279), a method wherein carbon black is used likewise the above and an amino group is introduced into the polymeric activating end (see JP-A-62-207342, JP-A-6-199923, JP-A-8-231658 and JP-A-8-225604) and the like.
However, almost of the improvements described in these patent articles are examined on the styrene-butadiene copolymer. Although the improving effect on the low heat buildup is recognized, when this copolymer is applied to a tread or the like of a heavy duty tire, it can not still be said that the low heat buildup is sufficient.
Further, in order to improve the low heat buildup and wear resistance, a high cis-polybutadiene rubber obtained by using a cobalt-nickel-neodymium catalyst has been used together with natural rubber from the old time. However, the low heat buildup is still insufficient likewise the above, so that the further improvement is required for satisfying the marketing needs.